This invention relates to web feeding systems and more particularly to a concave web guide which contacts the outside or convex surface of the web. The use of this web guide makes possible the design of a symmetrical, single capstan web feeding system wherein the characteristics of the system are independent of the direction of motion of the web and all elements of the system contact the same surface of the web. This invention is particularly applicable to magnetic tape drive systems.
The standard technique for changing the direction of motion of a web of material or a strip of magnetic tape is to wrap it around a circular web guide. To minimize the amount of friction between guide and tape, these guides typically are roller guides or air bearing guides. In either case, the tape is wrapped around the guide and the guide contacts the tape at the tape's inner surface. The result is a change of direction of the magnetic tape motion with no longitudinal compliance of the tape. It is not desirable for a guide to contact the oxide surface of the magnetic tape since loss of information may result. Therefore, most tape drives are designed so that all elements of the drive except the read/write head contact the non-oxide surface. Some examples of web guides are shown in U.S. Pat. No. 2,908,495.
A typical prior art tape drive comprises a capstan which pulls the magnetic tape across a read/write head. A tape guide is located on the opposite side of the head from the capstan to position the tape properly as it passes the read/write head. On either end of this feed system, the magnetic tape is fed through tapered buffer chambers or "pucker pockets," around additional guides, and through vacuum chambers before being spooled onto takeup reels. This system is not symmetrical since there is a capstan on one side of the head and a guide on the other. There are several disadvantages inherent in a non-symmetrical tape handling system.
One disadvantage is that the force available to accelerate the tape in a backward direction is in general less than in a forward direction. The system is designed so that, in the forward direction, the capstan pulls the tape across the read/write head. Since a typical inter-record gap on a magnetic tape is one half inch, and since the magnetic tape stops at about the center of this gap, it can be seen that the capstan must exert enough force on the magnetic tape to accelerate it up to operating speed within a quarter inch of longitudinal motion. The capstan, pucker pockets, and vacuum chambers are designed to achieve this capability in the forward direction. However, in the reverse direction, since the capstan cannot push the tape past the read/write heads, the tape must be accelerated by the longitudinal forces supplied by the pucker pocket and vacuum chamber. These forces are typically less and, therefore, the magnetic tape drive may not read properly in a reverse direction or at least comprises asymmetrical tape dynamics. Reverse direction reading is required in error checking and back spacing operations.
Another problem associated with non-symmetrical tape drive systems is that during long periods of high speed tape motion, the read/write head tends to be heated by friction with the tape. When the tape stops, the head may tend to stick to the tape, and considerable force is required to pull the tape from the head. In some systems, the force exerted by the pucker pocket and vacuum chamber in the reverse direction is not sufficient to pull the magnetic tape from the head, thus causing the system to halt. A solution to this problem is to have a "forward hitch" designed into the system. Since a capstan can exert more force on the tape than the pucker pocket and vacuum chamber can, when it is required to move the tape in a reverse direction, the tape will be initially pulled in a forward direction a slight amount to break the tape free from the head and then the tape will be moved in the reverse direction. Of course, this slows down the response time of the system.
A symmetrical tape drive could be designed wherein each side of one vacuum capstan would contact the leading and trailing portions of the magnetic tape, and wherein circular roller or air bearing tape guides would be used to guide the magnetic tape to the vacuum capstan. However, these tape guides would contact the oxide surface of the magnetic tape and therefore comprise a potential source of damage to the oxide and recorded information.